The present invention relates to a fuel cell system, and more particularly to an improvement in a technique for measuring AC impedance to detect an operating condition of a fuel cell.
It is known that the output of a fuel cell is affected by the internal condition of the fuel cell, for example the wetness of an electrolyte. The wetness of the electrolyte is commensurate with the complex impedance of the fuel cell, and therefore it has been proposed in the related art to monitor the operating condition of a fuel cell by applying an AC signal to the output of the fuel cell to detect both an amplitude ratio and a phase deviation between a current and a voltage and thereby calculate the complex impedance.
For example, Japanese Unexamined Patent Application Publication 2003-86220 describes a fuel cell system in which the complex impedance of a fuel cell is determined by applying a sine wave signal to an output signal of the fuel cell while varying the frequency from a high frequency to a low frequency, and the moisture condition of the fuel cell is estimated from a resistance component R1 that increases when the internal moisture content of the fuel cell is deficient and a resistance component R2 that increases when the internal moisture content is excessive. The resistance component R1 is measured by applying a high frequency sine wave signal, and the resistance component R2 is measured by applying a low frequency sine wave signal. Japanese Unexamined Patent Application Publication 2003-297408 describes a fuel cell system employing a similar technique, in which the water content of a measured gas is detected from either the voltage or the current of an electrochemical cell.
According to the conventional techniques described above, the internal moisture condition of a fuel cell can be learned indirectly by measuring the impedance of the fuel cell.